1. Field of the Invention
The present invention relates to a film-forming method by which a uniform alignment film of a helical polyacetylene can be formed by means of an LB method.
2. Description of the Related Art
An alignment film is of high utility value for various optical elements because the film has an optical anisotropy, and the film shows different optical characteristics depending on its polarization directions. In addition, the alignment film has been utilized for providing an initial alignment of liquid crystal in a liquid crystal display. In addition, the mobility of a polymer to be utilized in the present invention can be improved by aligning its molecules toward the direction in which electrodes are bridged. As described above, the alignment film has become a technology of importance in industry.
As general methods of producing an alignment body of an organic polymer, there are given methods such as a rubbing substrate method and a grating substrate method involving: producing a substrate provided with an alignment-regulating force in advance; and producing the alignment body of the polymer on the substrate. Also given are methods such as an electric field alignment method, a magnetic field alignment method, a flow alignment method, and an epitaxial growth method involving applying an external force upon formation of an aggregate of the polymer to produce the alignment body. Further given are methods such as a stretching-rolling alignment method, a friction transfer method, and an optical alignment method involving: producing a non-alignment body of the polymer in advance; and applying an external force to the body to align the body.
In addition to those alignment-controlling methods, a method of producing a polymer alignment film based on an LB method is known. The LB method is an abbreviation for a Langmuir-Blodgett method. In addition, a film formed by the Langmuir-Blodgett method is referred to as an “LB film.” The LB method or the LB film is detailed in “Langmuir-Blodgett Films” edited by G. Roberts, Plenum Press, New York (1990).
A monolayer of this kind which is floating on the surface of a liquid and exists at an interface between a gas and the liquid is referred to as a “Langmuir film” or an “L film.” The gas is generally air, and the liquid is generally water. A typical example of the monolayer formed at the gas-liquid interface is formed on the surface of water by placing amphiphilic molecules each formed of a hydrophobic portion and a hydrophilic portion in ordinary air. Those molecules are orientated on the water surface with their hydrophilic portions directed toward the water surface and their hydrophobic portions directed toward the air to form a monolayer film. A representative example of any such molecule is stearic acid formed of a hydrophobic alkyl chain and a hydrophilic carboxylic acid. A thin film can be formed by transferring such monolayer films onto a substrate one by one. The method is the LB method, and the film thus formed is the LB film.
FIGS. 1A and 1B each illustrate a standard trough used in the LB method. FIG. 1B illustrates a sectional view taken along the line 1B-1B of FIG. 1A. A liquid bath 100 for storing a liquid generally has a width and a length of about 10 cm to 1 m. The liquid bath is typically filled with a liquid. Water 101 is typically used as the liquid. The shape of the surface of the liquid stored in the trough is a quadrangle three sides of which are formed of side surfaces of the trough and the other side of which is provided with a movable barrier 102. The movable barrier 102 is provided for compressing molecules developed onto the water surface, and its movement is controlled from the outside. A general method for the feedback control of the movable barrier is such that the two-dimensional pressure of each molecule developed onto the water surface, that is, a surface pressure is kept constant.
Molecules 103 are developed onto the water surface. In the conventional LB method, amphiphilic molecules each having a hydrophilic portion and a hydrophobic portion are used. Representative examples of the molecules include stearic acid molecules. An LB film is formed on a substrate 104. The substrate serves as a base substrate upon formation of the LB film, and is preferably flat. A typical material for the substrate is, for example, an Si substrate or glass. The size of the substrate is generally, but of course not limited to, about several millimeters to about several tens of centimeters.
Film formation is performed in the conventional, general LB method as described below.
First, the movable barrier 102 is made sufficiently distant in a −x direction illustrated in the figures. In this state, the molecules to form a film are spread onto the surface of the water 101. At this time, the molecules are generally spread by: dissolving the molecules in a volatile solvent or a water-soluble solvent in advance; and spreading the solution onto the water surface with a syringe or the like. When amphiphilic molecules are used as the molecules, a single molecular layer is formed at a gas-liquid interface, that is, the surface of the water 101. Next, the movable barrier 102 moves in an x direction illustrated in the figures to compress the molecules 103 on the water surface toward the substrate 104. At this time, the area of the spread of the molecules 103 developed onto the water surface can be understood from the position of the movable barrier 102. More specifically, the total area occupied by the molecules 103 developed onto the water surface can be determined from the product of a length represented by reference numeral 105 and a length represented by reference numeral 106.
Meanwhile, the surface pressure of water can be determined by installing, on part of the water surface, a pressure sensor for detecting the surface pressure. A relationship between the area and the surface pressure upon movement of the movable barrier 102 can be determined by plotting both the area and the surface pressure. This is a graph called a π-A curve in the LB method, and FIG. 2 illustrates a typical π-A curve. In FIG. 2, the axis of abscissa indicates the area represented by the product of the lengths represented by reference numerals 105 and 106, that is, the total area occupied by the molecules 103 developed onto the water surface, and the axis of ordinate indicates the surface pressure. When the molecules are spread, the movable barrier 102 is sufficiently distant in the −x direction, and the molecules are sparsely developed onto the water surface. Therefore, the surface pressure is nearly zero. Such surface pressure corresponds to a region represented by reference numeral 201 in the π-A curve of FIG. 2. In the region, even when the movable barrier 102 is moved in the x direction illustrated in the figures, the molecules are merely present on the water surface in a sparse fashion, and the surface pressure is kept zero. As the movable barrier 102 is further brought close to the substrate 104 in the x direction illustrated in the figures, the surface pressure abruptly increases around the time point when an area occupied per one molecule on the water surface exactly coincides with the actual area of the molecule. This is because the molecules are exactly brought into a state of being closely packed all over the surface. The abrupt increase corresponds to a point of inflection represented by reference numeral 202 in the π-A curve of FIG. 2.
When the movable barrier 102 is further moved in the x direction from the point, the surface pressure abruptly increases as illustrated in FIG. 2. This is a state where the movable barrier 102 pushes the molecules closely packed all over the water surface, and the molecules are compressed while maintaining a single-layer state on the water surface. The state corresponds to a region represented by reference numeral 203 in FIG. 2. When the movable barrier 102 is further moved in the x direction, the surface pressure further increases, and finally, the monolayer film structure maintained on the water surface breaks. The point corresponds to a point of inflection represented by reference numeral 204 in FIG. 2. When the movable barrier 102 is further moved in the x direction, the pressure does not change largely and the area per one molecule reduces, which corresponds to a state where the molecules developed onto the water surface cannot maintain the monolayer structure and hence the structure breaks. The state corresponds to a region represented by reference numeral 205 in FIG. 2. When the compression is further continued, the surface pressure increases again. This is because the molecules turn into multiple layers or a mass.
In the LB method, a film is formed on the substrate 104 while the exact pressure at which the molecules form a monolayer molecular film on the water surface is applied. That is, the film is formed on the substrate 104 with the region represented by reference numeral 203 as described below. The position of the movable barrier is controlled so that the surface pressure at which the π-A curve in FIG. 2 reaches the region represented by reference numeral 203 may be applied. A typical surface pressure is generally about several millinewtons per meter to about several tens of millinewtons per meter.
In the state, the substrate 104 is moved in a vertical direction, i.e., a z direction illustrated in the figures. As a result, the monolayer molecular film on the water surface is transferred onto the substrate 104. When the molecules developed onto the water surface are low-molecular weight molecules each formed of a hydrophilic portion and a hydrophobic portion, the molecules are orientated so that the hydrophilic portions may contact water and the hydrophobic portions may be directed toward the air (gas phase). When the substrate is hydrophobic, the substrate is placed in the air first. Then, the substrate is immersed in water by being moved in a −z direction in FIG. 1A so that the substrate may be perpendicular. At that time, the monomolecular layer developed onto the water surface is transferred onto the substrate 104, which is hydrophobic, so that the surface of the substrate and the hydrophobic portion of each of the molecules may contact each other.
Next, a second layer is formed by moving the substrate in the z direction illustrated in the figures. Thus, a molecular film with its thickness controlled at a single-layer level can be formed. In addition, when the substrate is hydrophilic, the substrate is immersed in water in advance. Then, the substrate is moved from the inside of water in the z direction so that the first layer may be laminated. In this case, the first layer is formed so that the hydrophilic substrate and the hydrophilic portion of each of the molecules may contact each other. Hereafter, the second layer is laminated by moving the substrate in the −z direction. Generally employed in the LB method is such a method involving vertically moving the substrate placed perpendicularly to form a film as described above. Also available is a method involving placing the surface of the substrate parallel to the water surface and bringing the surface of the substrate into contact with the water surface from above the water surface to transfer the molecules on the water surface onto the substrate, or a method involving lifting upward the substrate placed below the water surface with its direction kept parallel to the water surface to transfer the molecules on the water surface onto the substrate in such a manner that the molecules are skimmed.
Although amphiphilic, low-molecular weight molecules such as arachidic acid are used in the most general LB method, a method involving the use of high-molecular weight molecules is also available. In this case, not all high-molecular weight molecules can form a monolayer film on a water surface or liquid surface, and only high-molecular weight molecules of which the monolayer film can be formed can be formed into a film by employing the LB method.
In an ordinary LB method or LB film, layers are laminated one by one, and hence the structure of the film can be controlled in a direction perpendicular to a lamination surface. Although a structure in one layer is hard to control in ordinary cases, the structure can be controlled to some extent by utilizing the flow of a monolayer film developed onto a water surface. FIGS. 3A to 3C each illustrate the movement of a monolayer film developed onto a water surface when the monolayer film on the water surface is transferred onto a substrate in the LB method, the movement being viewed from above an LB trough. The LB trough illustrated in each of FIGS. 3A to 3C is substantially the same as that illustrated in FIG. 1A, but differs from that illustrated in FIG. 1A in that a similar movable barrier 301 as well as the movable barrier 102 is provided. The movable barrier 301 moves in the direction opposite to that of the movable barrier 102 in the x direction. That is, when the movable barrier 102 moves in the x direction, the movable barrier 301 moves in the −x direction. In this case, the front side and rear side of the substrate 104 are symmetric with respect to each other, and hence films can be uniformly formed on both surfaces of the substrate 104.
FIGS. 3A to 3C each illustrate the movement on the water surface when a film is formed on the substrate 104, and time elapses as illustrated in FIG. 3A, FIG. 3B, and FIG. 3C in the stated order. In each figure, meshes and lattice points represent the same points on the water surface, and each represent the manner in which the corresponding point has moved. It can be understood that as the time elapses as illustrated in FIGS. 3A, 3B, and 3C in the stated order, the meshes deform by virtue of the flow of the water surface. In the vicinity of the substrate 104, the meshes each deform from a rectangular shape to a shape elongated in the x direction illustrated in the figures by virtue of the flow.
When a monolayer of a polymer is arranged on the water surface, the polymer is aligned so as to elongate in the x direction illustrated in the figures in the vicinity of the substrate by virtue of the flow as the time elapses. Since the layer is transferred onto the substrate while maintaining the alignment, a film in which the polymer is aligned is formed on the substrate. The polymer is aligned so as to elongate in the z direction illustrated in the figures on the substrate, and hence the film is transferred and formed. As described above, an alignment film can be formed by utilizing a flow in the LB method. The flow alignment is described in, for example, S. Schwiegk, et al., “With Regard to Origin of Main-chain Alignment of Rigid, Rod-like Polymer during Langmuir-Blodgett Process,” Thin Solid Films, 210 (1992) 6, or O. Albrecht, et al., “Control of Uniformity of Langmuir-Blodgett Film Using Lamination-induced Flow in Monolayer,” Thin Solid Films, 221 (1992) 276.